...as those email addresses won't work anymore anyway. This would make offboarding in WMF a little bit easier when it comes to finding accounts of staff (and not-anymore-staff).
Users themselves could do this at https://phabricator.wikimedia.org/settings/panel/email/
I realize it's only relevant when the email address is set as "primary", and that's the case only for three active Phab accounts.
I was not successful to contact the three affected users. Thus I ran UPDATE phabricator_user.user_email SET isPrimary = 0 WHERE address = "xxx"; UPDATE phabricator_user.user_email SET isVerified = 0 WHERE address = "xxx"; for the three affected accounts.